Adsorptive bubble separation (which includes froth flotation, flotation, bubble fractionation, dissolved air flotation, and solvent sublation) is a process in which a molecular, colloidal or particulate material is selectively adsorbed to the surface of gas bubbles rising through a liquid, and is thereby concentrated or separated. A commonly used type of adsorptive bubble separation process is froth flotation wherein the bubble-particle agglomerates accumulate on the liquid surface as a floating froth. The froth with adsorbed (i.e., attached or collected) particles is treated in one of several ways to collapse the froth and isolate the material. See for example, Flotation Science and Engineering, K. A. Mattis, Editor, pages 1 to 44, Marcel Dekker, New York, N.Y., 1995; and Adsorptive Bubble Separation Techniques, Robert Lemlich, Editor, pages 1 to 5, Academic Press, New York, N.Y., 1972.
This important process is commercially utilized in a wide range of applications including: isolation of minerals and metals from an ore-water slurry, dewatering of microalgae, yeast or bacterial cells, removal of oil from water, removal of ash from coal, removal of particles in waste-water treatment streams, purification of drinking water, and removal of ink and adhesives during paper recycling. In most applications, it is necessary to add reagents, known as “collectors”, which selectively render one or more of the species of particles in the feed hydrophobic, thereby assisting in the process of collection by the gas bubbles. It is also not unusual to add frothing agents to assist in the formation of a stable froth on the surface of the liquid. The process of admitting these various reagents to the system is known as conditioning. The feed for the adsorptive bubble separation process may be a mixture, dispersion, emulsion, slurry, or suspension of a molecular, colloidal and/or particulate material in a liquid and is referred to hereafter as the liquid-particle dispersion. When the liquid is water, as is usually the case, the feed may be referred to as an aqueous-particle dispersion.
Because of the importance of adsorptive bubble separation processes, there have been many attempts to improve the efficiency and selectivity of particle capture from an aqueous-particle dispersion in order to increase product yield and purity.
U.S. Pat. Nos. 4,668,382, 4,938,865, 5,332,100, and 5,188,726 (the contents of the entirety of each of which are incorporated herein by this reference) disclose an adsorptive bubble separation process and apparatus (commonly known as a “Jameson cell”) wherein an aqueous-particle dispersion enters the top of a vertical duct (downcomer) and passes through an orifice plate to form a high velocity, downward facing liquid jet. A gas, usually air introduced into the downcomer headspace, is dispersed into the mixture as the liquid jet impacts a foam column within the downcomer. The volume within the downcomer is referred to as the collection zone wherein most of the particles adsorb to the surface of the bubbles. The resulting gas-liquid-particle dispersion exits through the bottom of the downcomer into the separation zone where the bubbles separate from the tails (water and non-adsorbed materials). In the separation zone, the gas-liquid-particle dispersion has sufficient residence time to allow the tiny bubbles with collected particles to coalesce (combine and enlarge) and rise to the liquid surface forming a particle-rich, floating froth in the froth zone. The froth is collected by allowing it to float outward to the perimeter of the apparatus and overflow into an open launder (collection trough). Provisions are made in these patents to incorporate froth washing in the froth zone by introducing a liquid onto the froth from above thus creating a net downward liquid flow and washing the entrained gangue (undesired solid matter) and non-adsorbed particles away from the froth. This washing produces a purer froth, and therefore a more selective separation. In the design described in these patents, the washing occurs over the whole surface of the froth rather than in a focused region of the froth.
In addition, U.S. Pat. No. 4,668,382 (the contents of the entirety of which are incorporated herein by this reference) changes the configuration from a tank with vertical walls to converging walls so that the froth is squeezed (crowded) as it collects on the liquid surface. This allows for a higher froth depth than would normally occur, thus permitting better collection selectivity in the portion of froth overflowing into the collection launder. This design however requires an expensive fabrication process to make the converging sides.
U.S. Pat. No. 6,832,690 (the contents of the entirety of which are incorporated herein by this reference) also describes a method of squeezing the froth in a complex geometry, while U.S. Pat. No. 5,251,764 (the contents of the entirety of which are incorporated herein by this reference) describes a complex hydraulically-operated system. Froth zone surface fouling can be troublesome in these modifications of the original Jameson cell design.
In column flotation cells such as the MICROCEL™, U.S. Pat. Nos. 4,981,582 and 5,167,798; the Deister Column Cell, U.S. Pat. No. 5,078,921; and the Multistage Loop-Flow Flotation (MSTLFLO) column, U.S. Pat. No. 5,897,772 (the contents of the entirety of each of which are incorporated herein by this reference), the collection, separation, and froth zones and froth washing are combined in a tall, cylindrical tank, which is less effective and more expensive to construct. In these column flotation cells, the froth at the top of the column overflows into an outer launder that surrounds the column. Sometimes an additional central launder is added to increase the froth discharge area when it is necessary to achieve rapid removal of voluminous froth.
Mechanical flotation cells typically employ a rotor and stator mechanism for gas induction, bubble generation, and liquid circulation thus providing for bubble and particle collision. The ratio of vessel height to diameter, termed the “aspect ratio”, usually varies from about 0.7 to 2. Typically, four or more cells each having a centrally mounted rotor and stator mechanism are arranged in series. The liquid-particle dispersion is fed into the cell and air is sucked into the cell through a hollow shaft agitator. The air stream is broken by the rotating impeller, so that small bubbles are emitted from the end of the impeller blades. An auxiliary blower may also be used to provide sufficient gas flow to the cell. Rising bubbles together with attached particles form a froth layer on the top of the liquid surface. The froth layer overflows or is skimmed off mechanically from the top. Non-floated components are withdrawn from the bottom of the cell. Mechanical flotation cells are often used in mineral processing systems; however they have the disadvantage of large space requirements, long liquid residence times, and high power consumption.
For example, U.S. Pat. Nos. 4,425,232 and 4,800,017 (the contents of the entirety of which are incorporated herein by this reference) describe mechanical flotation separation utilizing a flotation cell provided with a rotor-stator assembly submerged in a slurry and in which rotor blades agitate the slurry thoroughly mixing the solids and liquid and introducing air to the mixture for aeration and generation of froth on the liquid surface. Particles of minerals attach to carrier air bubbles which are naturally buoyant and form the froth, this being the effective mechanism for mineral recovery. The floating froth is removed from the top of the slurry together with the attached mineral particles which are recovered as froth is collapsed and dewatered.
In all of these previously described processes, the desired particles that have prematurely disengaged (i.e., desorbed or detached) from the bubbles are inefficiently contacted with rising gas bubbles over the entire cross sectional area of the tank, thus lowering the chance of recapturing them. In addition, these designs typically have froth collection launders around the perimeter, which reduces the froth density as the froth spreads from the center outward (from low surface area to high surface area) thereby reducing the froth height and the selectivity of froth overflow.